Chemical vapour deposition precursors

ABSTRACT

Zirconium precursors for use in depositing thin films of or containing zirconium oxide using an MOCVD technique have the following general formula: Zr x (OR) y L z wherein R is an alkyl group; L is a  2 -diketonate group; x=1 or 2; y=2, 4 or 6; and z=1 or 2.

[0001] This invention concerns precursors for use in chemical vapourdeposition techniques, the production of electro-ceramic devicestherefrom, and their use in ferro-electric memories and I.R. detectors.

[0002] Metalorganic chemical vapour deposition (MOCVD) is a preferredmethod for depositing thin films, i.e. in the order of a few μm offerroelectric metal oxides, such as lead zirconate titanate,[Pb(Zr,Ti)O₃ or PZT and lanthanum-modified lead zirconate titanate.[(Pb,La)(Zr,Ti)O₃ or PLZT]. These electro-ceramic materials have a widerange of useful dielectric, ferroelectric, piezoelectric, pyroelectricand electrostrictive properties, giving rise to a variety of potentialapplications ranging from thermal imaging and security systems tointegrated optics and computer memories, e.g. DRAMS and non-volatileFERAMS.

[0003] The MOCVD technique involves transporting a metal as a volatilemetalorganic compound in the vapour phase followed by thermaldecomposition usually in the presence of oxygen on an appropriatesubstrate. The different types of substrate can be divided into threegroups, namely oxides, semiconductors and metals. Examples of suitableoxide substrates are SiO₂, SrTiO₃, MgO and Al₂O₃. Semiconductorsubstrates include silicon (Si) and germanium (Ge) and metal substratesmay be, for example, molybdenum (Mo) or tungsten (W). MOCVD has a numberof advantages over other deposition techniques, such as sol-gel orphysical vapour deposition. MOCVD offers potential for large-areadeposition, excellent film uniformity and composition control, high filmdensities and deposition rates and excellent conformal step coverage atdimensions less than 2 μm. Furthermore, MOCVD processes are compatiblewith existing silicon chemical vapour deposition processes used in ULSIand VLSI applications.

[0004] Precursors for MOCVD of electro-ceramic thin films are generallymetal β-diketonates, such as, for example, leadbis-tetramethylheptanedionate (Pb(thd)₂, or metal alkoxides. WO 96/40690discloses various metalorganic complexes of the formula MAyX, wherein Mis a y-valent metal, A is a monodentrate or multidentrate organic ligandcoordinated to M which allows complexing of MAy with X, y is a integerhaving a value of 2, 3 or 4 and X is a monodentrate, or multidentrateligand coordinated to M and containing one or more atoms independentlyselected from C, N, H, S, O and F. A may be a β-diketonate and X may betetraglyme, tetrahyrofuran, bipyridine, crown ether or thioether.

[0005] It is important that the precursors are volatile enough to betransported efficiently at source temperatures which are below theprecursor decomposition temperature. In other words, there should be anadequate temperature window between vaporisation and decomposition. Theprecursors used need to be compatible and not pre-react. They shoulddecompose to form the desired metal oxide in the same temperatureregion. Ideally, precursors have low toxicity and are stable underambient conditions.

[0006] Available metal alkoxide and metal -diketonate precursorsgenerally have only very low vapour pressures, so that high sourcetemperatures are required for MOCVD. For example, Pb(thd)₂ is typicallytransported at above 130° C. and Zr(thd)₄ at above 160° C. Inconventional MOCVD in which a carrier gas is passed through a precursorheld at a high temperature for the duration of the deposition process,this can lead to thermal ageing, i.e. decomposition of the precursorprior to transport into the reactor.

[0007] One way of avoiding this problem has been to use liquid injectionMOCVD, in which a solution of the precursor(s) in an appropriatesolvent, e.g. tetrahydrofuran, is evaporated and then transported to thesubstrate. In this way the precursor is only subjected to heating duringevaporation rather than for the duration of the MOCVD process.

[0008] For ease of handling and volatility, toxicity and decompositioncharacteristics, the optimum precursor combination for MOCVD of PZT isPb(thd)₂, Zr(thd)₄ and either Ti(OPr^(i))₄ or Ti(OPr^(i))₂(thd)₂.However, there is a problem with using Zr(thd)₄, in that it is toostable, making it difficult to control the stoichiometry of PZT duringliquid delivery MOCVD. In particular, there is a large differencebetween the decomposition temperature of Zr(thd)₄ and the most usefullead precursor Pb(thd)₂. This results in a significant differencebetween the temperatures for diffusion (or mass-limited) oxide filmgrowth between the two precursors and the need to use high substratetemperatures to decompose the Zr(thd)₄ source leads to a loss of leadfrom the PZT films by evaporation.

[0009] Zirconium aLkoxides, such as Zr(OPr^(i))₄ and Zr(OBu^(t))₄ arepredicted to be much less thermally stable than Zr(thd)₄ but are highlyair and moisture sensitive making them difficult to manufacture in pureform and too unstable for long term storage.

[0010] An object of this invention is to provide alternative Zrprecursors for use in MOCVD, especially for depositing PZT and PZLT.

[0011] Another object of the invention is to provide an improved methodof depositing zirconium containing metal oxides in thin films.

[0012] According to a first aspect of this invention there is provided azirconium precursor suitable for use in MOCVD having the formula

Zr_(x)(OR)_(y)L_(z)

[0013] wherein R is an alkyl group

[0014] L is a β-diketonate group,

[0015] x=1 or 2

[0016] y=2, 4 or 6, and

[0017] z=1 or 2

[0018] According to a second aspect of the invention there is provided amethod of depositing thin films of or containing zirconium oxide usingmetalorganic precursors in an MOCVD technique, wherein the zirconiumprecursor has the formula

Zr_(x)(OR)_(y)L_(z)

[0019] wherein R is an alkyl group

[0020] L is a β-diketonate group.

[0021] x=1 or 2

[0022] y=2, 4 or 6, and

[0023] z=1 or 2

[0024] The preferred alkyl groups R are branched chain alkyl groups,preferably having less than 10 carbon atoms, more preferably having 1 to6 carbon atoms, especially iso-propyl and tertiary-butyl groups.

[0025] The preferred β-diketonate groups L include those of the generalformula

[0026] wherein R¹ and R² are the same or different and are straight orbranched, optionally substituted, alkyl groups or, optionallysubstituted, phenyl groups. Examples of suitable substituents includechlorine, fluorine and methoxy.

[0027] Examples of suitable β-diketonate groups for use in precursors ofthe invention include the following: R¹ R² CH₃ CH₃ acetylacetonate(acac) CF₃ CH₃ trifluoroacetylacetonate (tfac) CF₃ CF₃hexafluoroacetylacetonate (hfac) CH₃ C(CH)₃ dimethyiheptanedionate (dhd)C(CH)₃ C(CH)₃ tetramethyffieptanedionate (thd) CH₃ CF₂CF₂CF₃heptafluoroheptanedionate (fhd) C(CH)₃ CF₂CF₂CF₃ heptafluorodimethyl-(fod) octanedionate CF₂CF₂CF₃ CF₂CF₂CF₃ tetradecafluoro- (tdfnd)nonanedionate C(CH₃)₃ CF₃ trifluorodimethyl- (tpm) CF₃ CF₂CF₃octafluorohexanedionate (ofhd) C(CH₃)₃ CF₂CF₃ pentafluorodimethyl- (ppm)heptanedionate CF₃ CF₂CF₂CF₃ decafluoroheptanedionate (dfhd) C(CH₃)₃CH_(2CH) _(2CH) _(2OCH) ₃ dimethylmethoxy- (dmmod) octanedionate CCL3CH₃ trichioropentanedionate (tclac) Ph Ph diphenyipropanedionate (dpp)

[0028] In one preferred embodiment of the invention the zirconiumprecursor has the following formula:

Zr(OR)₂L₂

[0029] wherein R and L are as defined above.

[0030] Typical examples of such zirconium precursors include Zr(OP^(i))₂(thd)₂ and Zr(OBu^(t) ₂(thd)₂

[0031] These compounds are believed to be particularly suitable for usein the method according to the invention, especially in liquid injectionMOCVD.

[0032] In another preferred embodiment of the invention, the zirconiumprecursor has the following formula:

Zr₂(OPr^(i))₆(thd)₂

[0033] Again this compound is believed to be particularly suitable foruse in the method of the invention, especially in liquid injectionMOCVD.

[0034] Compounds of the invention may be produced by reaction of anappropriate zirconium alkoxide with an appropriate β-diketone.

[0035] The method of the invention is particularly useful for depositingon a substrate thin films, i.e. in the order of up to 5 μm of leadzirconate titanate (PZT) using a zirconium precursor according to theinvention with a lead precursor, such as Pb(thd)₂ or lanthanum-modifiedlead zirconate titanate (PLZT). Typical substrates include SiO₂, Si,SrTiO₃, MgO, Al₂O₃, Ge, Mo and W.

[0036] According to a further aspect of the present invention there isprovided a method of forming an electro-ceramic device comprising thesteps of depositing a lower conducting electrode onto a substrate,depositing a film layer of or containing zirconium oxide onto saidelectrode and depositing an upper or further conducting electrodethereon, wherein the zirconium oxide layer is formed from the zirconiumprecursor having the formula:

Zrx(OR)yLz

[0037] wherein R is an alkyl group;

[0038] L is a β-diketonate group;

[0039] x=1 or 2;

[0040] y=2, 4 or 6; and

[0041] z=1 or 2.

[0042] The lower conducting electrode and upper conducting electrode ispreferably a metal, for example, platinum. The substrate is preferably asilicon wafer or circuit. An electro-ceramic device formed by thismethod is particularly suitable for use in ferro-electric memories andinfra-red detectors.

[0043] This invention will be further described with reference to theaccompanying drawings, in which:

[0044]FIG. 1 shows ¹H NMR spectrum for the product prepared in Example 1below;

[0045]FIG. 2 shows mass spectrometry results for the product prepared inExample 1 below;

[0046]FIG. 3 shows ¹H NMR spectrum for the product prepared in Example 2below;

[0047]FIG. 4 shows mass spectrometry results for the product prepared inExample 2 below;

[0048]FIG. 5 is a plot of growth rates against substrate temperatureachieved by MOCVD using the products of Examples 1 and 2;

[0049]FIG. 6 shows a plot of growth rates against substrate temperatureachieved by MOCVD using a lead precursor;

[0050]FIG. 7 shows the likely chemical structure of Zr₂(OPr^(i))₆(thd)₂;

[0051]FIG. 8 is a plot of the growth rates against substrate temperatureachieved using the precursor Zr₂(OPr^(i))₆(thd)₂; and

[0052]FIG. 9 is a lateral cross-sectional view of an electro-ceramicdevice according to one embodiment of the present invention.

[0053] This invention will now be further described by means of thefollowing Examples.

EXAMPLE 1

[0054] Preparation of zirconium di-isopropoxybis-tetramethylheptanedionate.

[0055] 74 g of tetramethylheptanedionate were dissolved with stirring in1 litre of hexane in a 2 litre flask.

[0056] 75 g of zirconium isopropoxide iso-propanol adduct were added tothe flask and the mixture brought to reflux for 1 hour. The flask wascooled and the contents filtered through a pad and reduced in volume todampness using a Buchi roto-evaporator. The residue was redissolved in300 ml of hexane, clarified through a filter pad, stripped to halfvolume and 300 ml of dry isopropanol were added. The resultant solutionwas reduced in volume to 150 ml and set aside to crystallize thenfiltered off. The crystals were air dried or gently Buchi dried untilthe odour of isopropanol was removed.

[0057] The resultant product was relatively air stable, very soluble inhexane and tetrahydrofuran, fairly soluble in ethanol and less inisopropanol. NMR and mass spectral analysis results for the product areshown in FIGS. 1 and 2 of the accompanying drawings respectively and themicroanalysis results are as follows: Analysis % C % H Calculated 58.439.04 Found 56.86 8.30

[0058] These results indicate that the product had an approximatestoichiometry of Zr₂(OPr^(i))₂thd₂.

EXAMPLE 2

[0059] Preparation of zirconium di-tertiary-butoxybis-tetramethylheptanedionate

[0060] 72 g of tetramethylheptanedione were dissolved with stirring in 1litre of hexane in a 2 litre flask. 74 g of zirconium tertiary butoxidewere added to the flask (a slightly exothermic reaction) and the mixturebrought to reflux for 1 hour. The flask was cooled and its contentsfiltered through a pad before being reduced in volume to 200 ml using aBuchi roto-evaporator and set aside to crystallize. The resultingcrystals were filtered off and dried in air or gently Buchi dried tillthe odour of hexane was removed.

[0061] The product was air stable, very soluble in hexane andtetrahydrofuran, fairly soluble in ethanol and less in isopropanol.

[0062] NMR and mass spectral analysis results for the product are shownin FIG. 3 and 4 of the accompanying drawings respectively. The resultsof the elemental microanalysis are given below: Analysis % C % HCalculated 59.11 9.20 Found 58.66 8.70

[0063] These results indicate that the product had an approximatestoichiometry of Zr₂(OBu^(t))₂thd₂.

EXAMPLE 3

[0064] Deposition of ZrO₂ thin films.

[0065] Thin films of ZrO₂ have been deposited by liquid injection MOCVDwith both Zr(OPr^(i))₂(thd)₂ and Zr(OBu^(t))₂(thd)₂ in concentrations of0.09M in tetrahydrofuran. An evaporator temperature of 200° C. was usedwith argon flow of 4 litres/min and oxygen flow of 100-300 sccm. Growthrates achieved at different substrate temperatures are shown in FIG. 5of the accompanying drawings.

[0066] The suitability of either of these ZrO₂ precursors for use with atypical lead precursor, such as Pb(thd)₂ can be established from FIG. 6of the accompanying drawings which shows film lead oxide, including PbO₂growth rates from this lead precursor at different substratetemperatures. As can be seen from FIGS. 5 and 6 both the Zr and Pbprecursors provide optimum growth rates over a similar range ofsubstrate temperatures, i.e. from about 450-525° C.

[0067] It is believed that these Zr precursors are relatively stable toair and moisture due to having six-fold co-ordination around the Zrcentre, in contrast to the coordinately unsaturated Zr(OR)₄ compounds.

EXAMPLE 4

[0068] The product from Example 1 was recrystallized from n-hexane. Theresultant product had the stoichiometry of Zr₂(OPr^(i))₆(thd)₂ as shownin FIG. 7 of the accompanying drawings.

EXAMPLE 5

[0069] Synthesis of Zr₂(OPr^(i))₆(thd)₂

[0070] Zirconium isopropoxide (2.93 g, 7.56 mmol) was dissolved intoluene (50 cm³) and tetramethylheptanedionate (1.58 cm³, 7.56 mmol) wasadded. The solution was stirred at reflux for 1 hour after which timeall volatiles were removed in vacuo to yield a cubite solid. The whitesolid was re-dissolved in toluene (20 m³) and left to stand at 0° C.overnight. Colourless crystals of Zr₂(OPr^(i))₆(thd)₂ were filtered off.

EXAMPLE 6

[0071] Synthesis of Zr₂(OPr^(i))₆(thd)₂

[0072] Zirconium isopropoxide (2.97 g, 7.25 mmol) was dissolved in nhexane (20 ml) and tetramethylheptane-dionate (3.02 cm³, 14.5 mmol) wasadded. The solution was stirred at reflux for 1 hour after which timeall volatiles were removed in vacuo to yield a white solid. This wasre-dissolved in n-hexane (10 cm³) and left to stand overnight. Thecrystallisation process was repeated four times to yield colourlessrhombohedral cyrstals which gave the single ray crystal X-ray structureof Zr₂(OPr^(i))₆(thd)₂. A proposed chemical structure forZr₂(OPr^(i))₆(thd)₂ is shown in FIG. 7 of the accompanying drawings.

[0073] Zr₂(OPr^(i))₆(thd)₂ is believed to be suitable for deposition ofthin films of ZrO₂ by liquid injection MOCVD.

EXAMPLE 7

[0074] Growth rates achieved using the precursor of Example 6.

[0075] Zr₂(OPr^(i))₆(thd)₂ has proved suitable for the deposition ofthin films of ZrO₂ by liquid injection MOCVD. The films were grown usinga 0.1 molar solution of Zr₂(OPr^(i))₆(thd)₂ in tetrahydrofuran. Anevaporator temperature of 200° C. was used with a precursor injectionrate of 3.5 cm³ hr⁻¹, an argon flow of 3000-5000 cm³ min⁻¹ and an oxygenflow of 1000-2000 cm³ min⁻¹. The growth rates achieved at differentsubstrate temperatures are shown in FIG. 8 of the accompanying drawings,and indicate that ZrO₂ growth occurs over a significantly widertemperature range than is achievable with other precursors such as Zralkoxides or Zr(thd)₄.

[0076] It is believed that the novel Zr₂(OPr^(i))₆(thd)₂ source is moresuitable than existing Zr precursors for the MOCVD of Pb(Zr,Ti)O₃ andrelated ferro-electric materials at low substrate temperatures and ofyttria-stabilised zirconia at more elevated temperatures.

[0077] The zirconium precursors according to the present invention maybe used in the preparation of electro-ceramic device 2, as shown in FIG.9 of the accompanying drawings. A lower conducting electrode 6, such asplatinum is deposited onto a substrate 4, such as silicon wafer orcircuit and a film layer 8 of a zirconium oxide is formed thereon usingthe zirconium precursor of the present invention. An upper conductingelectrode 10, which may also be platinum, is then deposited onto thezirconium oxide layer by appropriate deposition techniques. Theelectro-ceramic device may be used, for example, in ferro-electricmemories or infra-red detectors, such as those used in security lights.

1. A zirconium precursor suitable for use in MOCVD having the formula:Zr_(x)(OR)_(y)L_(z) wherein R is an alkyl group, L is a β-diketonategroup, x=or 2, y=2, 4 or 6, and z=1 or 2
 2. A zirconium precursor asclaimed in claim 1, wherein R is a branched chain alkyl group.
 3. Azirconium precursor as claimed in claim 2, wherein R has less than 10carbon atoms.
 4. A zirconium precursor as claimed in claim 3, wherein Rhas 1 to 6 carbon atoms.
 5. A zirconium precursor as claimed in claim 4,wherein R is selected from isopropyl and tertiary butyl groups.
 6. Azirconium precursor as claimed in any one of claims 1 to 5, wherein theβ-diketonate group L has the following formula:

wherein R¹ and R² are the same or different and are selected fromstraight or branched, optionally substituted, alkyl groups andoptionally substituted, phenyl groups.
 7. A zirconium precursor asclaimed in claim 6, wherein said optional substituents are selected fromchlorine, fluorine and methoxy.
 8. A zirconium precursor as claimed inany one of claims 1 to 7 having the formula Zr(OR)₂L₂
 9. The zirconiumprecursor Zr(OPr^(i))₂(thd)₂, wherein thd is a tetramethylheptanedionategroup.
 10. The zirconium precursor Zr(OBu^(t))₂(thd)₂, wherein thd is atetramethylheptanedionate group.
 11. The zirconium precursorZr₂(OPr^(i))₆(thd)₂ wherein thd is a tetramethylheptanedionate.
 12. Amethod of depositing thin film of or containing zirconium oxide usingmetalorganic precursors in an MOCVD technique, wherein the zirconiumprecursor is as claimed in any one of claims 1 to
 11. 13. A method asclaimed in claim 12, wherein one of the precursors is a lead precursor.14. A method as claimed in claim 13, wherein the lead precursor isPb(thd)₂.
 15. A method as claimed in claim 12, wherein the substrate isselected from SiO₂, Si, SrTiO₃, MgO, Al₂O₃, Ge, Mo and W.
 16. The use ofa zirconium precursor as claimed in any one of claims 1 to 11 in thedeposition of thin film of or containing zirconium oxide by an MOCVDtechnique.
 17. A method of forming an electro-ceramic device comprisingthe steps of depositing a lower conducting electrode onto a substrate,depositing a film layer of or containing zirconium oxide onto saidelectrode and depositing an upper or further conducting electrodethereon wherein the zirconium oxide layer is formed from a zirconiumprecursor as claimed in any one of claims 1 to
 11. 18. A method asclaimed in claim 17, wherein the lower and/or upper conducting electrodeis a metal.
 19. A method as claimed in claim 18, wherein the metal isplatinum.
 20. A method as claimed in claim 17, 18 or 19, wherein thesubstrate is selected from a silicon wafer or circuit.
 21. Anelectro-ceramic device formed by the method of claim
 17. 22. Anelectro-ceramic device as claimed in claim 21 for use in ferro-electricmemories.
 23. An electroceramic device as claimed in claim 21 for use inan infra-red detector.